Due both to demographic change and to developments in medical science, the number of surgical procedures involving prosthesis implantation is rising rapidly. The more obvious examples of prosthetic devices are hip or knee replacements and false teeth. Other less well-known examples are stents, heart valves, bone screws and plates and spinal fixators.
Prosthesis must be tolerated by the patient and not altered in time. Materials that may be suitable for each type of prosthesis are subjected to precise specifications. Indeed, if the prosthesis is a dental implant or a hip replacement the specifications will be very different. The most important requirements are mechanical properties similar to those of bone to allow the transfer constraints between bone and prosthesis, chemical resistance to corrosion, chemical inertia in relation to the environment and biocompatibility. These properties must be controlled to maintain the integrity of used materials. The human body is an aggressive and corrosive environment mainly because of concentrations of chloride ions (113 mEq/l in blood plasma and 117 mEq/l in the interstitial fluid, which is sufficient to corrode metallic materials) and dissolved oxygen. For dental implants, conditions are even tougher since the saliva contains more sulfur products that make it still more corrosive. The term “biocompatibility” is defined by the Dorland's Medical Dictionary as the quality of not having toxic or injurious effects on biological systems. This encompasses both the material and host responses to an implant. The host response to an implant can be highly complex and is often linked to the material response. It is also dependent on the anatomical position of the implant. For a material to be biocompatible, it should not elicit any adverse host reactions to its presence. Inflammation and encapsulation phenomena may occur when the prosthesis suffer from low biocompatibility.
Typically, prosthetic devices are made of inorganic (metal, alloys, ceramic and glass) and/or polymeric materials.
It is a fact that most pure metals and alloys are chemically unstable in many everyday environments due to their tendency to corrode. In the complex environment of the human body, metals and alloys are subject to electrochemical corrosion mechanisms, with bodily fluids acting as an electrolyte. While alloys such as stainless steel may appear to be highly stable and are widely used for kitchenware, eating utensils and jewelry, there are many situations that can cause severe corrosion of this material, and it is not the best choice for use in prosthetic devices.
Compared to inorganic materials, polymeric materials have certain advantages: they are lightweight, corrosion resistant, they can be directly shaped by molding and offer design freedoms. Over the past 4 years, the price of steel and non-ferrous metals grew faster than polymers and they require also less energy to be implemented. Among various existing polymers, only some of them have been used in the prosthesis industry so far, mainly because of their biocompatibility. Examples of such polymers are polymethyl methacrylate, polystyrene, poly(ether ether ketone).
The Applicant has found that specific polymeric compositions feature surprisingly outstanding properties and in particular a good biocompatibility and impact resistance. These outstanding properties may be useful in certain demanding applications, such as articles used as prosthetic devices.
Polyarylenes, especially polyphenylenes, exhibit some outstanding performance properties, including exceptionally high strength, stiffness, hardness, scratch resistance, friction and wear properties and dimensional stability. Unfortunately, polyarylenes while offering an exceptionally high level of strength and stiffness have some limitations in toughness-related properties, in particular in terms of impact resistance (as typically characterized by standard notched and unnotched Izod tests) and elongation properties. They have also limitations in melt processability due to their high viscosities, and tend to be anisotropic when melt fabricated under high shear such as during injection molding. Also, they have some limitations in chemical resistance.
Poly(aryl ether ketone)s, especially poly(ether ether ketone)s, exhibit also some outstanding properties, including exceptionally high melting point, excellent chemical resistance (including environmental stress cracking resistance) and excellent thermal stability. They have also high strength, stiffness, although somewhat lower than that of polyarylenes, and very good elongation properties. On the other hand, like polyarylenes, they have some limitations in terms of impact resistance.
Poly(aryl ether sulfone)s in general and poly(biphenylether sulfone)s in particular are typically amorphous and do not melt crystallize. Due to their high strength and heat resistance, certain poly(aryl ether sulfone)s may be used in high-stress environments where other polymers may degrade or may otherwise be unsuitable. Poly(aryl ether sulfone)s provide good chemical resistance, toughness, lightness and strength, processability in the melt phase including injection molding and extrusion; and ease of colorability.
Polymer blends have been widely taught and employed in the art. As broad as this statement may be, the blending of polymers remains an empirical art and the selection of polymers for a blend giving special properties is, in the main, an Edisonian-like choice. Certain attributes of polymer blends are more unique than others. The more unique attributes when found in a blend tend to be unanticipated properties. According to Zoller and Hoehn, Journal of Polymer Science, Polymer Physics Edition, vol. 20, pp. 1385-1397 (1982): “Blending of polymers is a useful technique to obtain properties in thermoplastic materials not readily achieved in a single polymer. Virtually all technologically important properties can be improved in this way, some of the more important ones being flow properties, mechanical properties (especially impact strength), thermal stability, and price ( . . . ). Ultimately, the goal of such modeling and correlation studies should be the prediction of blend properties from the properties of the pure components alone. We are certainly very far from achieving this goal.” Moreover, in the field of miscibility or compatibility of polymer blends, the art has found predictability to be unattainable, even though considerable work on the matter has been done. According to authorities: “It is well known that, regarding the mixing of thermoplastic polymers, incompatibility is the rule and miscibility and even partial miscibility is the exception. Since most thermoplastic polymers are immiscible in other thermoplastic polymers, the discovery of a homogeneous mixture or partially miscible mixture of two or more thermoplastic polymers is, indeed, inherently unpredictable with any degree of certainty”; for example, see P. J. Flory, Principles of Polymer Chemistry, Cornell University Press, 1953, Chapter 13, page 555.
U.S. Pat. No. 4,662,887 describes a high modulus prosthetic device made of poly(ether ether ketone).
U.S. 2007111165 describes a prosthetic dental device made of a thermoplastic polymer including a poly(aryl ketone), such as poly(ether ether ketone) (PEEK), polymethylmethacrylate (PMMA), poly(aryl ether ketone) (PAEK), poly(ether ketone) (PEK), poly(ether ketone ether ketone ketone) (PEKEKK), poly(ether ketone ketone) (PEKK), and/or polyetherimide (PEI), polysulfone (PSU), and polyphenylsulfone (PPSU).
All materials used in the prior art prosthetic devices still suffer from a limited impact resistance, which is a key property for such application where such materials are submitted to various and harsh conditions.
There remains thus a strong need for a prosthetic device presenting a superior balance of properties, including part or preferably all of the following ones:                very high strength;        very high stiffness;        good elongation properties;        good melt processability (in particular, good injection moldability);        high chemical resistance;        good biocompatibility;        outstanding impact resistance, as possibly characterized by a standard no-notch IZOD test (ASTM D-4810).        
In fact, there is a specific need to improve the mechanical properties of the existing prosthetic devices and in particular their impact resistance while at least maintaining their biocompatibility.